1. Field of the Invention
The present invention relates to a magnetic sensor having a free magnetic layer, a nonmagnetic conductive layer, and a pinned magnetic layer, and in particular, to a magnetic sensor that pins the magnetization of the pinned magnetic layer by means of a uniaxial anisotropy of the pinned magnetic layer itself.
2. Description of the Related Art
Recently, most magnetic heads mounted in magnetic recording and playback devices include a spin valve magnetic sensor that uses the giant magnetoresistive (GMR) effect.
The spin valve magnetic sensor includes a ferromagnetic film called a pinned magnetic layer, a ferromagnetic soft magnetic film called a free magnetic layer, and a nonmagnetic film called a nonmagnetic conductive layer. The nonmagnetic conductive layer is disposed between the pinned magnetic layer and the free magnetic layer.
The magnetization of the free magnetic layer is aligned in a particular direction by a longitudinal bias magnetic field from, for example, a hard bias layer composed of a hard magnetic material. The magnetization of the free magnetic layer is sensitive and is changed in response to an external magnetic field generated from a recording medium. On the other hand, the magnetization of the pinned magnetic layer is pinned in a direction that is perpendicular to the magnetization direction of the free magnetic layer.
The relationship between the changes of magnetization direction of the free magnetic layer and the pinned magnetization direction of the pinned magnetic layer changes the electrical resistance. A leakage field from the recording medium is detected by changes of voltage or current based on the change of the electrical resistance.
In the known spin valve magnetic sensor, the pinned magnetic layer is formed on an antiferromagnetic layer composed of an antiferromagnetic material such as PtMn. An exchange coupling magnetic field is created between the pinned magnetic layer and the antiferromagnetic layer, thereby pinning the magnetization of the pinned magnetic layer.
The exchange coupling magnetic field generated at the boundary between the antiferromagnetic layer and the pinned magnetic layer can be large enough to prevent the change of the magnetization direction of the pinned magnetic layer, due to the application of a magnetic field during the manufacturing process or a leakage field from the recording medium. Furthermore, the antiferromagnetic layer itself does not generate an external magnetic field; therefore, this structure simplifies the design of the magnetic sensor.
However, the antiferromagnetic layer must have a thickness of about 200 Å so that the exchange coupling magnetic field generated at the boundary between the antiferromagnetic layer and the pinned magnetic layer has a sufficient intensity to provide the above benefits.
An antiferromagnetic layer having a large thickness disposed in the laminated component of a magnetic sensor mainly causes a shunt loss of a sense current. In order to achieve a high recording density on the recording medium, the output of the magnetic sensor must be improved. However, the shunt loss of the sense current prevents improvement of the output of the magnetic sensor.
Furthermore, shield layers composed of a soft magnetic material are disposed on the magnetic sensor and under the magnetic sensor in order to effectively read recording signals to be detected. Moreover, in order to achieve a high linear recording density on the recording medium, the distance between the top shield layer and the bottom shield layer must be small. However, an antiferromagnetic layer having a large thickness prevents the distance between the top shield layer and the bottom shield layer from being small.
Referring to FIG. 12, a magnetic sensor that does not have the antiferromagnetic layer has been proposed. The magnetic sensor pins the magnetization of the pinned magnetic layer by means of a uniaxial anisotropy of the pinned magnetic layer itself.
The magnetic sensor shown in FIG. 12 include a multilayer film T having, from the bottom, a bottom gap layer 1, an underlayer 2, a pinned magnetic layer 3 having a synthetic ferrimagnetic structure, a nonmagnetic conductive layer 4, a free magnetic layer 5, and a protective layer 6 in that order. The pinned magnetic layer 3 includes a first magnetic sublayer 3a, a second magnetic sublayer 3c, and a nonmagnetic interlayer 3b disposed therebetween. Furthermore, at both sides 7 of the multilayer film T, bias base layers 8, hard bias layers 9, and electrode layers 10 are formed.
The magnetic sensor shown in FIG. 12 does not have the antiferromagnetic layer that is overlapped with the pinned magnetic layer 3. According to this magnetic sensor, the magnetization of the pinned magnetic layer 3 is pinned in the Y direction shown in the figure by means of the uniaxial anisotropy of the pinned magnetic layer 3 itself. Accordingly, the shunt loss can be small compared with a known magnetic sensor having the antiferromagnetic layer. Thus, the output in detecting the magnetic field of the magnetic sensor can be improved by 20% to 30%. Furthermore,. the distance between the top shield layer and the bottom shield layer, which are disposed on and under the magnetic sensor, respectively, can be small, thereby achieving a higher linear recording density on the recording medium.
Japanese Unexamined Patent Application Publication Nos. 8-7235 (pp. 8 and 9, FIG. 5) and 2000-113418 (pp. 7 and 8, FIGS. 4, 5, 6, and 7) disclose the magnetic sensor shown in FIG. 12.
The magnetic sensor described in Japanese Unexamined Patent Application Publication No. 8-7235 includes a buffer layer 62, i.e., an underlayer, composed of tantalum (Ta), and a pinned ferromagnetic layer 70 disposed thereon. The pinned ferromagnetic layer 70 includes a first cobalt (Co) film 72, a second cobalt (Co) film 74, and a ruthenium (Ru) film 73 disposed therebetween. The magnetization of the first cobalt (Co) film 72 and the second cobalt (Co) film 74 is pinned by each of the anisotropy fields. The first cobalt (Co) film 72 and the second cobalt (Co) film 74 are antiferromagnetically bonded to each other. The first cobalt (Co) film 72 and the second cobalt (Co) film 74 are magnetized in antiparallel directions.
As described above, the magnetic sensor described in Japanese Unexamined Patent Application Publication No. 8-7235 includes the buffer layer composed of tantalum having cobalt (Co) films thereon. Unfortunately, this structure cannot appropriately pin the magnetization direction of the pinned ferromagnetic layer 70. Japanese Unexamined Patent Application Publication No. 2000-113418 also points out this problem.
The magnetic sensor described in Japanese Unexamined Patent Application Publication No. 2000-113418 was invented in order to solve the problem in Japanese Unexamined Patent Application Publication No. 8-7235. According to this magnetic sensor, ferromagnetic layers in a laminated ferrimagnetic pinned layer are composed of CoFe or CoFeNi, thereby improving the induced anisotropy.
Furthermore, Japanese Unexamined Patent Application Publication No. 2000-113418 discloses an underlayer composed of Ta, the underlayer being disposed under the laminated ferrimagnetic pinned layer. Referring to experimental results in which the presence of the Ta underlayer is compared (see FIGS. 4, 5, 6, and 7 in Japanese Unexamined Patent Application Publication No. 2000-113418), when using CoFe alloy as the ferromagnetic layers, a magnetic sensor that does not have the Ta underlayer has a larger magnetoresistance change and a larger coercive force than those of the magnetic sensor that has the Ta underlayer.
In Japanese Unexamined Patent Application Publication No. 2000-113418, CoFe alloy is used as the ferromagnetic layers, and the ferromagnetic layers have a positive magnetostriction in order to increase the induced anisotropy of the laminated ferrimagnetic pinned layer.
One factor to pin the magnetization of the self-pinning type pinned magnetic layer is the uniaxial anisotropy due to magnetoelastic energy of the pinned magnetic layer. In particular, one may optimize the magnetostriction of the pinned magnetic layer. However, Japanese Unexamined Patent Application Publication No. 2000-113418 does not consider or discuss optimization of the magnetostriction of the pinned magnetic layer let alone disclose a specific construction to optimize the magnetostriction of the pinned magnetic layer.